Antibody (immunoglobulin, Ig) class switch causes B lymphocytes to switch from producing IgM to producing IgG, IgA or IgE, which improves the ability of the antibody to remove pathogens and bacterial toxins from the body. Class switching occurs by an intrachromosomal DNA recombination event that must be carefully controlled in order to avoid aberrant recombination with other chromosomes (translocations). However, occasional translocations do occur. Sometimes they result in B lymphomas or myelomas due to translocations of the Ig genes to a chromosomal site encoding an oncogene, e.g. c-myc, which induces aberrant expression of the c-myc gene. This application proposes to investigate how DNA double-strand breaks (DSBs) are introduced into the mouse c-myc gene and the role of activation-induced cytidine deaminase (AID) in the process of c-myc-IgH translocations. In Aim 1, we will investigate the hypothesis that reactive oxygen species (ROS) induce DNA lesions that lead to DSBs in the c-myc gene, and also stimulate the deamination activity of AID, as deamination occurs by an oxidation reaction. In Aim 2, we will investigate whether AID has additional activities besides its deaminase activity that contribute to creating c- myc-IgH translocations. In Aim 3, we will investigate the role of the 10 amino acids at the C terminus of AID, which is required for class switch recombination but not for somatic hypermutation, and has been shown to repress c-myc-IgH translocations. We will examine the hypothesis that DNA repair proteins involved in switch recombination are recruited by the C terminus. PUBLIC HEALTH RELEVANCE: This project investigates the events that initiate the generation of B cell lymphomas and myelomas. We will study how DNA breaks are introduced into the c-myc oncogene when normal mouse B cells are induced to undergo the normal process of antibody class switching in response to activation signals. These DNA breaks can result in translocations of the c-myc oncogene to the immunoglobulin (Ig) heavy (H) chain gene locus, which increases expression of the c-myc gene, and this can lead to cell transformation and malignancy. We will study whether the protein that is required for initiating DNA break formation in the IgH genes during antibody class switching (activation-induced cytidine deaminase, AID) is required for introducing DNA breaks into the c-myc gene, and whether AID has additional roles that lead to the translocations between the IgH genes and c-myc genes. We will also investigate the function of AID itself during normal class switching, as this affects translocations between the c-myc gene and the IgH locus.